As illustrated in U.S. Pat. No. 3,846,783 issued to Aspell et al on Nov. 5, 1974, it is known to provide a printing pager. The pager is digitally addressed and a message is subsequently transmitted in FSK (frequency shift keying) form to the addressed pager. This requires a convenient encoding method and unit as well as circuits at the pager for receiving transmitted signals, decoding a particular address, decoding the FSK signals and printing the message.
As described in the Aspell et al patent, one of the problems with this type system is end of message decoding. It will be apparent that the pager should be in part shut off after the transmission of a message, if for no other reason than to save battery power. In the Aspell et al system this is accomplished by transmitting a separate end of message code, which is then detected at the pager. Unlike this system, the present invention provides as one of its features, that no end of message code need be separately sent. Rather the absence of message signals are detected by recognizing a predetermined nonzero integration for the logic level "1" and "0" pulses making up the decoded message. When a message is being received each character is preceded by a start bit which is at a logic level "0". This keeps the integration level per character less than a predetermined level when characters are present. However, when the message terminates, only "1's" will appear (or "0's", depending on the logic). When integrated over a predetermined period of time, the signal representing the sum of the "1's" will grow until a predetermined threshold representing an "end of message" is reached. When this threshold is exceeded various section of the pager may be shut down. This is accomplished without the need for a separate end of message signal. Thus power and complexity can be saved by use of this simple technique.
Another power saving feature which is one of the subjects of this invention is the incorporation of a one tone FSK decoder. As is common, two FSK tones are utilized in formulating the FSK code. These two tones are typically detected separately at a receiver and utilized to reconstruct the coded message. In the present system, however, only one of the two tones is detected, it being assumed that the other tone is present in the absence of the detected tone. Reliability of this system is established by a unique logic and filter circuit utilizing two monostable multivibrators and a shift register clocked by the input signal. The multivibrators set up a "window" in which the detected pulse must fall to be recognized, with the multivibrator output forming the "data" input to the shift register. A 3 out of 4 decode unit even further increases the reliability of the one-tone decode system.
Another feature of the subject invention involves encoding of the message to be transmitted. Encoding in one embodiment may be done in either a hand held hard copy or soft copy encoder, accoustically coupled via a telephone line to the high power transmitter used in the paging system. Thus, the message is locally generated and transmitted by phone.
In one encoder embodiment, the encoder includes an LED dot matrix alpha/numeric display (soft copy) and a message entry keyboard. It is a feature of this invention to provide a single logic circuit to decode the characters entered at the keyboard while at the same time providing timing (strobing) signals for the columns of the dot matrices of the display. This is accomplished by providing a binary counter which is continually cycled through all its states. The states of the counter correspond to different characters. A binary decoder decodes the output of the counter and provides pulses at its many output terminals in a predetermined sequence. These sequential pulses form both the display strobing signals and also indicate the state of the counter and thus the character represented by its count. These signals are applied to the keyboard such that with the depression of a particular key, the counter is read out at the right time to dump the character indicated by the key. Thus the decoder output pulses serve the two distinct functions of keyboard decoding and display strobing, thereby saving circuity and battery power
In the hard copy encoder a printed tape indicates the encoded message. The message is first entered into a memory and then read out onto the tape so that there need be no synchronism between the keyboard decoding and the tape imprinting. This saves considerable circuitry. When the memory of the encoder is full, it is an additional feature of the subject invention that the memory is continuously recycled resulting in continuous tape imprinting to provide a highly visible memory overload alarm. This technique can also be provided in the soft copy encoder, with continuous precession of the message indicating memory overload.
In a yet still further aspect of this invention, an improved digital address decoder is provided which includes an improved bit recognition technique. In the prior art, the digital address is loaded in its entirety into a register where it is then compared with an address stored in another register. This requires a number of comparators equal to the number of bits in the address. Unlike the prior art, the subject system compares the incoming address bit by bit as it arrives, through a single comparator, with a predetermined address carried in a shift register shifter in synchronism with the incoming digital signal. This system reduces complexity and performs a real time decoding function.
In order to describe the above feature and how they interrelate, several entire paging systems will be discussed, including hard and soft copy pagers, hard and soft copy encoders and a "print-on-the-fly" thermal tape printing system useful in the hard copy embodiments.
It is therefore an object of this invention to provide an improved hand held paging system in which a message transmitted to a recipient is stored at the pager for readout at the convenience of the recipient.
It is another object of this invention to provide a paging system which is both inexpensive and makes few demands on the portable power supply contained within the pager.
It is yet another object of this invention to provide an improved end of message recognition system not requiring a separately generated end of message signal.
It is a further object of this invention to provide a soft copy pager which may be utilized both for the receipt of a transmitted message and for the encoding of a message to be transmitted thereby combining in one unit the two functions.
It is yet still further object of this invention to provide a one tone FSK decoding system.
It is still further object of this invention to provide a message entry unit in which the printout actuation is independent of the keyboard actuation and in which memory overload is indicated by the free running of the printing tape.
It is another object of this invention to provide a message encoder utilizing a single logic circuit to provide display strobing signals and keyboard decoding signals, with the same signals being used for both functions.
It is another object of this invention to provide an improved digital address decoding system.